Self-assembling monolayer (SAM) films have been used as building units for constructing multilayer structures and as modifiers of surface properties. SAMs are prepared by selective adsorption of compounds at solid fluid interfaces to construct organized oriented compact monolayers of good quality and having a thickness ranging from about 1 nanometer to about 3 nanometers. The molecular self-assembly process takes place as a layer-by-layer process, which is based on the spontaneous adsorption of either nonionic polymers, polyanions or polycations from dilute aqueous solutions onto surfaces that carry a functional group or a charge opposite to that of the depositing polymer. Selective adsorption of these polyelectrolytes is alternated to form a bilayer assembly and leads to the formation of multilayer assemblies. The molecules which are typically used for constructing the first monolayer have a terminal polar group and a non-polar functional group at either the other end of the molecule or somewhere within it.
Past patterning techniques using thin films as only passive resists have not provided patterned SAMs with uniform patterns or the high resolution required for potential applications such as full color flat displays, membrane separation, electroluminescent devices, conducting and insulating circuits, optical and nonlinear optical devices, and multi-element chemical sensors. Therefore, it is desirable to prepare uniform and consistently patterned SAMs which may be used to fabricate such devices.
Sagiv in U.S. Pat. No. 4,539,061 provided a method for the chemical modification of a monolayer coated solid surface in order to introduce polar sites for anchoring an additional monolayer on top of an activated monolayer. His method provided a surface with desired surface properties. In this process ordered multilayer assemblies were prepared by direct adsorption of certain types of bifunctional molecules onto suitable solids via a sequence of chemical operations performed on the film coated solid. The key step in the process comprised first forming a monolayer of molecules having a terminal polar group at one end and a non-polar one at the other end of the molecule or at any other position along the molecule. After the first compact monolayer is formed by self-assembly on the solid substrate, the monolayer is activated by introducing polar sites for anchoring an additional monolayer on top of the activated one. The additional monolayer is of similar nature to the first layer. However, it should be noted that these SAMs have inherent limitations which prevent them from being used to build multilayered films. After seven bilayers were formed, further growth or layering of the film was terminated. This occurred because growth of the film required 100% reactivity at each stage in the process. In addition, the SAMs that resulted from this process are not patterned. Instead the SAMs coat the entire surface of the substrate and must later be patterned using etch or other techniques.
Balachander et al. ("Monolayer Transformation by Nucleophilic Substitution: Applications to the Creation of New Monolayer Assemblies", Langmuir, The ACS Journal of Surfaces and Colloids, November 1990, volume 6, number 11, pp. 1621-1627) present a series of new trichlorosilyl-terminated surfactants which have been used to create a set of variously functionalized SAM surfaces. Transformations of these surfaces, with a focus on the use of nucleophilic substitution reactions for the creation and interconversion of surfaces with amine- and thiol-containing functionality were reported. These reactions were used to create new surface functionality and bridged monolayer structures. The monolayers were prepared by immersion of the substrate into a beaker containing a long-chain alkyl trichlorosilyl solution. The substrate was then quickly withdrawn from the solution and washed with CHCl.sub.3 and water and cleaned in hot CHCl.sub.3 in a Soxhlet extractor for 15 minutes. Depending on the surfactant used, the surfaces were transformed into various Y- and Z-terminated surfaces. However, these surface treatments formed only a monolayer structure that covered the entire surface of the substrate.
Decher et al. (U.S. Pat. No. 5,208,111) describe one or more multi-layer elements applied to supports. The elements consist of a modified support having an even surface, in which modification means the application of ions or ionisable compounds of the same charge over the entire area. One or more layers made of organic materials are applied to the support and each layer contains ions of the same charge. The ions of the first layer have the opposite charge of the modified support. In the case of several layers, each further layer has the opposite charge of the previous layer. The layer elements are applied to supports by applying the individual layers from solutions of organic materials. This results in one or more multi-layer elements covering an entire surface of the support. Decher et al. fail to provide a patterned molecular self-assembly nor do they a discuss a method for preparing patterned molecular self-assemblies.
Rubner et al. (U.S. Pat. No. 5,536,573) proposed a molecular self-assembly of electrically conductive polymers. Their process is driven by the attractions developed between a positively charged p-type doped conducting polymer and a negatively charged polyion or water soluble, nonionic polymer. Like Decher et al., they fail to disclose a patterned molecular self-assembly or process for preparing patterned molecular self-assemblies.
Chan et al. ("Polymeric Self-Assembled Monolayers, 3, Pattern Transfer by Use of Photolithography, Electrochemical Methods, and an Ultra thin Self-Assembled Diacetylenic Resist", Journal of the American Chemical Society, 1995, volume 117, pp. 5875-5876) disclose that a substrate can be patterned using a diacetylenic, self-assembled monolayer (SAM) resist and photolithographic and electrochemical methods. In this instance, the diacetylenic SAM is used as a negative photoresist wherein the image of a transmission electron microscope (TEM) minigrid is transferred into a gold (Au) substrate. In their process, a SAM composed of close-packed HS--(CH.sub.2).sub.10 C.tbd.CC.tbd.C(CH.sub.2).sub.10 COOH molecules is placed on an unpatterned Au/Cr/Si surface. A minigrid is placed in contact with the SAM to form an assembly. The entire assembly is then exposed to UV light, which induces polymerization in the unmasked regions of the SAM. Next, the unpolymerized portion of the SAM resist is selectively desorbed using an electrochemical reductive stripping method. Selective stripping is possible because the polymeric SAM is sufficiently insoluble and strongly bound to the surface through multiple Au/S and van der Waals interactions that it survives potential excursions that remove monomeric organomercaptan SAMs. Resist removal results in a negative image of the mask, which can be elaborated by etching the grid image into the Au surface with an oxygen saturated 1 M KOH plus 10 mM KCN aqueous solution. In this particular method, the entire substrate is first coated with a monolayer film and then the monolayer film is removed and later etched to leave the negative image of the mask. However, it was found that the lateral dimensions of the hexagonal regions formed by this process were found to be somewhat less than those of the original mask.
Hammond ("New method makes patterned polymer films"; Chemical and Engineering News, Oct. 6, 1997) proposed a way to control the adsorption process horizontally and vertically. In her method, substrates are prepared using a microcontact printing technique. In this process a "rubber stamp" containing a pattern is "inked" with 16-mercaptohexadecanoic acid and pressed onto a gold-coated silicon substrate. The stamping process imprints the gold substrate with lines of the carboxylic acid functionality. The imprinted substrate is then immersed in a solution of oligo-(ethylene glycol)-terminated alkanethiol to cover the gold regions left exposed. The now-patterned gold surface serves as a molecular template for polyions. In this process, the pattern is established on the previously treated areas of the surface instead of the exposed surface of the support. The problem with this technique is that the surface must be completely inked before the assembly is prepared. As with many rubber stamping processes, the chance of the "ink" not covering the whole stamp is great, thus the ability to provide uniform and repeated patterns in manufacturing is greatly reduced.
Adair et al. (U.S. Pat. No. 5,501,877) propose a method for creating patterned films on substrates. The method comprises creating a photomask pattern on the substrate, adsorbing seed particles of the film material from a colloidal suspension on the substrate, followed by chemical vapor deposition (CVD) to create a thin film on the selected areas. This method is applicable to high surface energy material capable of film formation through CVD techniques on a substrate. Adair et al. fail to mention the application of this process to molecular self-assemblies. In fact, this technique is to be used with CVD which is a much different technique than molecular self-assembly technique. The CVD technique will not produce the high resolution patterns required for many of the previously mentioned applications. Nor will it give a uniform, pin-hole free film. In addition, the thickness of the films from the CVD technique is limited to a few hundred nanometers.
An object of the present invention is to provide a uniform, patterned molecular self-assembly.
Another object of the present invention is to provide a uniform, multi-patterned molecular self-assembly.
Another object of the present invention is to provide a process for preparing a uniform, patterned molecular self-assembly where the pattern is made by selectively adsorbing the molecular self-assembly onto an exposed surface of a support.
A further object of the present invention is to provide a process for preparing a uniform, multi-patterned molecular self-assembly where two different patterns are selectively adsorbed onto exposed surfaces of a support.